In 1066 the Normans invaded England.
They came from France.
They brought with them the French language.
And they changed the English language forever.
The Anglo-Saxon dialects of the Germanic languages have been infused with French words but remained otherwise a model Germanic language. Since the French-speaking Normans were the rulers, it is mostly courteous words (and the word "courteous" is a hint where this is going) that were replaced with French words, while familiar words remained Germanic. In many cases both the French and Germanic words survived and even today it is usually improper to use the Germanic word in police society while the French word should be used instead.
One word that was replaced completely was the Angl0-Saxon word for "doubt".
The original Anglo-Saxon word for doubt is a direct cognate of the German word for "doubt", "Zweifel".
Recall that a Germanic <t> corresponds to an original Indo-European <d> and, ultimately, a German <tz> (spelt "z").
The German word "Zweifel" is cognate with Latin "dubio".
The "z" corresponds to the "d", the "w" to the "u" and the "f" corresponds to "b". This mirrors the German word "zwei" and its Latin cognate "duo" (as well as its English cognate "two", which has "t" instead of "z").
A "doubt" is in fact a possibility that there is more than one outcome for something. It is quite literally a "two-ness" of a thing and the word "doubt" ultimately derives from the Indo-European word for "two".